For me, to whisper softly doesn't necessarily convey the notion of se dire des motsdoux. It just means to whisper in a very low tone. I might "whisper softly" in church or at a play so as not to disturb other people.

I suppose it's "nothings" because the words and phrases only mean something to the participants (or just the woman, La C!), OR because the words themselves are not of great import - it is the feelings behind them and the closeness they engender which matter.